Damping circuit for excavator multi-motor load sharing swing drive

ABSTRACT

The swing drive motor armatures on an excavator are connected in alternate fashion with the armatures of their associated generators to form a sandwiched loop. Sets of nodes which are at the same potential under steady state conditions are thus formed in the loop and a damping resistor is connected between a pair of these equal-potential nodes to conduct current when oscillations occur in the mechanical portion of the swing drive. As a result, the swing drive motors generate output torques which oppose and damp these oscillations. Alternative arrangements are shown in which four motor armatures and four generator armatures are connected in a single sandwiched loop, or in which a pair of sandwiched loops are formed with two motor armatures and two generator armatures in each loop.

United States Patent [1 1 Jones DAMPING CIRCUIT FOR EXCAVATOR 1MULTI-MOTOR LOAD SHARING SWING DRIVE [75] Inventor:

[73] Assignee: Bucyrus-Erie Company, South Milwaukee, Wis.

22 Filed: Mar. 10, 1972 21 Appl. No.: 233,633

Byron M. Jones, New Berlin, Wis.

[ Apr. 23, 1974 4/1924 Santuari 318/87 9/1919 Ferris 318/88 X 1 ABSTRACTThe swing drive motor armatures on an excavator are connected inalternate fashion with the armatures of their associated generators toform a sandwiched loop. Sets of nodes which are at the same potentialunder steady state conditions are thus formed in the loop and a dampingresistor is connected between a pair of these equal-potential nodes toconduct current when oscillations occur in the mechanical portion of theswing drive. As a result, the swing drive motors generate output torqueswhich oppose and damp these oscillations. Alternative arrangements areshown in which four motor armatures and four generator armatures areconnected in a single sandwiched loop, or in which a pair of sandwichedloops are formed with two motor armatures and two generator armatures ineach loop.

1 Claim, 4 Drawing Figures SPEED AND DIRECTION c o N TROL 55'PATENTEUAPR 23 mm SHEET 2 [IF 3 AND PATENTEDAPR 2 m 3.806; 780

SHEET 3 [1F 3 CONTROL SPEED AND ' DIRECTION DAMPING CIRCUIT FOREXCAVATOR MULTI-MOTOR LOAD SHARING SWING DRIVE BACKGROUND OF THEINVENTION The field of the invention is electro-mechanical drive unitsand particularly for swing drive machinery for large excavators.

Any mechanical drive system has a natural resonant frequency orfrequencies at which it will oscillate when excited. These mechanicaloscillations subject the components of the system to cyclic loadingwhich repeatedly stress the components and which may result in theirpremature failure.

This problem is particularly acute in the swing drive units of largeexcavators. Due to backlash in the gear train linking the swing drivemotor and the swing gear, oscillations occur each time the system isstarted, stopped or reversed. These mechanical oscillations subject theshafts and gears in the swing drive to particularly high and repeatedtransient torques which considerably limit their useful life. Inaddition, stress oscillations occur when the swing drive is operated ator driven through certain swing speeds. Such oscillations occur whennatural mechanical resonances of the system are excited by the rate atwhich gear teeth mesh or engage at various points in the gear train.

Mechanical damping means have been devised and installed in swing drivesystems to reduce the number and magnitude of stress cycles. However,these known damping means are quite large, expensive and are themselvessubject to premature fatigue failures. The latter problem is particularyprevalent when attempts are made to cut the cost of mechanical dampingmeans or reduce its size to meet specific space limitations. As aresult, in large excavating machines the life of the drive unit is oftenmore effectively and economically extended by increasing the size, orcapacity of those components which have, through experience, been foundto be particularly subject to fatigue failure.

SUMMARY OF THE INVENTION The present invention relates to means fordamping the oscillations which occur in the mechanical portion of anelectro-mechanical drive unit, which means comprises a modification inthe electrical portion of the drive unit. Specifically, the inventionincludes the connection of the drive motors and their associatedgenerators in a loop configuration and connecting a damping resistor, orresistors, between equal-potential nodes in the loop. It has beendiscovered that the speed of each drive motor oscillates in response tomechanical vibrations, and that such speed oscillations in turn causecorresponding variations in motor armature voltage and current. Thephase relationship of these voltage and current variations depends onnumerous factors such as the physical location of the drive units withrespect to each other and the nature of the mechanical oscillationswhich are occurring. Regardless of the phase relationship of thesevoltage and current variations, however, the present invention operatesin response to the mechanical oscillations to vary the torque output ofeach drive motor in such a manner that the drive motors respond tooppose torque variations in the mechanical portion of the drive units tothereby damp the mechanical oscillations.

A general object of the invention is to provide a means for dampingoscillations in an electromechanical drive system, which means does notrequire extensive mechanical alterations to present structures. Nomodifications are made to the mechanical portion of the drive system;instead, the arrnatures of the drive motors and their associatedgenerators are connected in a sandwiched loop; that is, each motorarmature is connected in series between two generator armatures and viceversa. Sets of equal-potential nodes are thus formed around the loop atthe connection points of the armatures, and a damping resistor isconnected between any two equal-potential nodes of a set. Under steadystate operating conditions, the voltage at the equal-potential nodes ofeach set is the same and no current flows through the damping resistor.However, when transient torque demands are made on the drive motors,such as when the motor shafts are oscillating, a considerable ripplevoltage is developed across the damping resistor. Current is thus causedto flow through the damping resistor. This current flow is in such adirection that more current is supplied to the swing motors which areoperating at lower speeds with the resulting increase in their outputtorque. On the other hand the torque output of those swing motors whichare speeding up is decreased with the result that the electrical portionof the drive unit damps, or opposes rapid fluctuations in torque.

Another object of the invention is to provide a means for dampingoscillations in swing drive units presently in use. The alterations andequipment additions necessary to implement the present invention requirea minimal amount of labor and space. Generally, a reconnection of themotors and their associated generators, along with the addition of oneor more resistors is all that is required.

The foregoing and other objects and advantages of the invention willappear from the following description. In the description reference ismade to the accompanying drawings which form a part hereof, and in whichthere is shown by way of illustration preferred embodiments of theinvention. Such embodiments do not necessarily represent the full scopeof the invention and reference is made to the claims herein forinterpreting the breadth of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view, with parts cutaway, of a drag line excavator which incorporates the present invention,

FIG. 2 is a view in elevation, with parts cut away, of a drive unitwhich incorporates the present invention,

FIG. 3 is an electrical schematic diagram of a first embodiment of theinvention, and

FIG. 4 is an electrical schematic diagram of a second embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An excavator 1 of the type towhich the present invention has been successfully applied is shown(somewhat schematically) in FIG. 1. The excavator 1 is of the drag linetype and has a lower works to which a pair of feet 2 and 3 are attachedfor walking. A circular shaped swing gear 4 is mounted on the lowerworks with its central axis 5 located on the machine center line. Arevolving frame 6 is rotatably attached and mounted atop the lowerworks, and a boom 7 is pivotally attached to the front of the revolvingframe 6 to support a bucket (not shown in the drawings) which ismanipulated to perform excavating functions.

Mounted on the revolving frame 6 and securely fastened thereto, are fourswing drive units 8, 9, 10 and 11. The swing drive units 8-11 aredisposed above the swing gear 4 and around an approximately 120 sectorof its circumference. Referring to FIG. 2, each drive unit 8-11 includesa direct-current, stabilized, shunt wound swing motor 12 mounted to thetop of a gear case 13. Contained within the gear case 13 is a doublereduction divided train transmission 14 which links the swing motor 12with a vertical swing shaft 15 that extends downward through the bottomof the gear case 13 to the vicinity of the swing gear 4 on the lowerworks. A pinion gear 16 is attached to the lower end of the swing shaft15 and engages the gear rack 4. The motors 12 on the respective driveunits 8-11 are operated simultaneously to rotate their respective piniongears 16 and to thereby revolve, or swing the frame 6 and attached boom7 with respect to the lower works. The swing motors 12 are matched todivide the load equally between them under normal steady state operatingconditions, and as an example of one embodiment each motor 12 is rated620 horsepower at 1,000 rpm and 460 volts.

Although the mechanical portion of each drive unit 8-11 is not part ofthe present invention, the mechanism described above generates theproblem which is solved by the present invention. To appreciate themagnitude of this problem, the physical size of the mechanical elementsinvolved is illuminating. For example, the vertical swing shaft 15 isapproximately 12 feet long and 16 inches in diameter. The pinion gear 16is approximately 2 feet in diameter, and 8 barrels of oil are requiredto lubricate the transmission 14 of each drive unit 8-1 1.

The mechanical resonances which occur in this mechanical drive areparticularly severe when the swing motor speed passes through the 500rpm range. It has been calculated that the frequency at which thisresonance occurs (3.4 cycles per second) corresponds to the naturaltorsional vibration frequency of the swing shaft 15 and its connectedinertias. It has also been discovered in connection with making thisinvention that this frequency corresponds to the frequency with whichthe teeth on the pinion gear 16 engage the teeth on the swing gear 4when the swing motors 12 are rotating at about 500 rpm. The result ofthese oscillations is the cyclic stressing of all the mechanicalcomponents in the swing drive units 8-11. This can eventually lead tofatigue failure in any one of these components.

Referring now to FIG. 3, the electrical connection of the swing driveunits 8-11 is shown schematically. In the description of the electricalcircuits to follow, the swing motor 12 on each drive unit 8-11 will beidentified separately. The swing motor 12 of the first drive unit 8 isdesignated generally by the dashed line 19 and includes an armature Mconnected in series with an interpole, or commutating field winding 17.A separately excited motor field winding 18 is also magnetically coupledto the armature M Connected in series with the armature M, is thearmature G of a first generator, designated generally by the dashedlines 20. The first generator 20 includes a series field winding 21 anda commutating field winding 22 both connected in series circuit with thearmature 0,. Connected in series with the first generator 20 is a secondswing motor 12 attached to the second drive unit 9 and designatedgenerally by the dashed lines 23. Second swing motor 23 includes anarmature M, a separately excited field winding 24 and a commutatingfield winding 25 connected between the armature M and the firstgenerator 20. The other lead of the armature M connects to an armature Gof a second generator, designated generally by the dashed lines 26. Thesecond generator 26 includes a series field winding 27 and a commutatingfield winding 28, both connected in circuit between the armature G andcommutating field winding 17 of the first swing motor 19. A first loop29 is thus formed and includes, in order, the first motor armature M thefirst generator armature G the second swing motor armature M and thesecond generator armature G The loop 29 formed by the alternateconnection of motor armatures and generator armatures is termed herein asandwiched loop. The generators 20 and 26 also include shunt windings,the connection and function of which is described hereafter.

The first sandwiched loop 29 includes two sets of equal-potential nodes.The first set includes equalpotential nodes 30 and 31 locatedrespectively at the connection between the first motor 19 and firstgenerator 20, and the connection between the second motor 23 and secondgenerator 26. This second set includes equal-potential nodes 32 and 33located respectively at the connection between the first generator 20and second motor 23, and the connection between the second generator 26and first motor 19. The term equalpotential nodes refers to points inthe sandwiched loop 29 which assume the same voltage or potential levelunder steady state operating conditions. In other words, under steadystate conditions, the voltage developed across the nodes 30 and 32equals the voltage developed across the nodes 31 and 33, and therefore,according to Kirchoffs law the voltage level at node 30 equals that atnode 31 and the voltage level at node 32 equals that at node 33.

A second sandwiched loop 34 is formed with the remaining two swingmotors and their associated generators. Specifically, a third swingmotor 12 attached to the third drive unit 10 and designated generally bythe dashed lines 35, has an armature M connected in series with thearmature G, of a third generator designated generally by the dashedlines 36. The third swing motor 35 also includes a separately excitedfield winding 37 and a commutating field winding 38 connected in serieswith the armature M The third generator 36 includes a series fieldwinding 39 and a commutating field winding 40, both connected in serieswith the generator armature G A fourth swing motor attached to thefourth drive unit 1 1 and designated generally by the dashed lines 41 isconnected between the third generator 36 and the armature G of a fourthgenerator designated generally by the dashed lines 42. The fourth motor41 includes an armature M, connected in the loop 34 in series with acommutating field winding 43. The motor 41 also has a separately excitedfield winding 44. The fourth generator 42 has a series field winding 45connected in series with a commutating field winding 46 between thegenerator armature G, and the third motor 35. Two sets ofequal-potential nodes are thus formed. The first set includes nodes 47and 48 located respectively between the third motor 35 and fourthgenerator 42 and between the third generator 36 and fourth motor 41. Thesecond set includes nodes 49 and 50 located respectively between thethird motor 35 and third generator 36 and between the fourth motor 41and fourth generator 42.

The steady state speed and torque produced by the four swing motors 19,23, 35, and 41 is controlled by a speed and direction control circuit 51connected to the first loop 29. The speed of a d-c motor isproportionate to the voltage developed across its armature and thetorque output of the motor is proportionate to its armature current.Speed and torque information is fed to the speed and directional controlcircuit 51 by a first set of leads 52 connected across the firstgenerator armature G, and its series field winding 21, and a second setof leads 53 connected across the commutating field windings 22 and 25 ofthe first generator 20 and second swing motor 23. The voltage generatedacross the leads 53 is proportionate to the current flowing in the firstsandwiched loop 29 and is, therefore, proportionate to the torquegenerated by the first and second motors l9 and 23. The voltagegenerated across the leads 52 is proportionate to the voltage dropacross the first and second motor armatures M and M and is, therefore,proportionate to the speed of their shafts.

There are numerous commercially available speed and torque controlcircuits. in the preferred embodiment, a control circuit such as thatdisclosed in US. Pat. No. 3,518,444 issued to D. E. Barber on June 30,1970, is used. The control circuit 51 operates to regulate the level ofthe current flowing in both the first and second sandwiched loops 29 and34, and to limit the voltage developed by the four generators 20, 26, 36and 42. The generators are stabilized shunt wound d-c machines rated 560kilowatts at 480 volts and 1,200 rpm. Control of the generators isaccomplished by regulating the current through both a first set ofgenerator shunt field windings 54, 55, 56 and 57 and a second set ofgenerator shunt field windings 58, 59, 60 and 61. The two sets of fieldwindings are connected to the output of the control circuit 51 and awinding in each set is magnetically coupled to one of the generatorarmatures G G G or 6,. To drive the motors 19, 23, 35 and 41 in onedirection, current is supplied to the first set of generator fieldwindings 54-57. The magnitude of this current determines the voltagedeveloped by the generators 20, 26, 36 and 42, which in turn determinesthe speed and torque output of the swing motors for any given load.Similarly, when the direction of swing is to be reversed, current issupplied to the second set of generator field windings 58-61 to reversecurrent flow in the sandwiched loops 29 and 34. Because the drive units8-11l are the same and the mechanical load is shared equally by them,the current flowing in each sandwiched loop 29 and 34 and the voltagelevels developed at corresponding nodes are substantially the same understeady state conditions. Thus, even though speed and torque feedbackinformation is obtained from the first loop 29, control is effectivelymaintained over both sandwiched loops 29 and 34 with a single speed anddirection control circuit 51.. This economy of using a single controlcircuit is magnified further when larger numbers of drive units andloops are required.

When oscillations occur in the swing drive mechanisms, the load is nolonger shared equally and a certain amount of control is lost over thecurrent flowing in the second sandwiched loop 34. Also, the speed anddirection control circuit 51 regulates average total current flow in thefirst sandwiched loop 29, and it does not respond instantaneously totransient voltage and current variations caused by mechanicaloscillations. However, when such transient, or rapidly varying, currentsflow in the first and second sandwiched loops 29 and 34, the voltagepattern at the equal-potential nodes is substantially altered.Specifically, the voltage levels at the equal-potential nodes are nolonger equal, but instead, vary with respect to one another in responseto the imposed mechanical oscillations. The mechanical oscillationsimposed on the swing drive motor shafts 15 cause, at any point in time,some of the motors to speed up and others to slow down. The speedvariations are reflected as voltage variations across each motorarmature. And herein lies an important discovery which the presentinvention implements. By connecting damping resistor(s) between pairs ofequal-potential nodes, the voltage differentials which are generatedbetween equal-potential nodes by oscillations in the mechanical portionsof the drive units, cause current to flow through the dampingresistor(s) to effectively damp the oscillations. Specifically, a firstdamping resistor 62 is connected between the equal-potential nodes 32and 33 in the first sandwiched loop 29 and a second damping resistor 63is connected between the equalpotential nodes 47 and 48 in the secondsandwiched loop 34. During oscillation, the damping resistors 62 and 63each operate nearly instantaneously to divert armature current away fromone swing motor and provide more armature current to the swing motorwhich is operating at the slower speed. As a result, oscillatory drivingtorques are generated by the swing motors 19, 23, 35 and 41, whichtorques oppose the oscillatory load torques imposed by the mechanicalsystem. The implementation of this discovery can be improved by theconnection of additional damping resistors between other pairs ofequal-potential nodes. For example, a third damping resistor 64 (shownin phantom lines) can be added to the first sandwiched loop 29 betweenthe equal-potential nodes 30 and 31, and a fourth damping resistor 65(shown in phantom lines) can be added to the second sandwiched loop 34between the equalpotential nodes 49 and 50. In systems having additionalequal-potential nodes, additional damping resistors can be added. Also,additional sandwiched loops similar to the loops 29 and 34 can be formedwhere additional drive units are required.

The damping resistors are of relatively low value, preferably beingcomparable in resistance to the equivalent impedance of the circuit towhich the damping resistor is connected. In the preferred embodiment,the damping resistors have values of 0.03 ohms and power ratings of 5kilowatts.

The improvement which is obtained by the addition of the presentinvention to existing excavators is substantial. When the swing drive ona Bucyrus-Erie Model 1500 W dragline was connected as described above,the following results were obtained during normal digging operations.

Swing Shaft Torque (Ft. Lbs.)

350,000 400,000 450,000 500,000 Prior Art Numerous l20 26 4 7 WithInvented Damper 24 2 0 The table indicates the number of instances inwhich the swing shaft torque exceeded the levels indicated during theoperating interval. These results clearly illustrate the reduction incycle loading of the swing machinery provided by the present invention.

An alternative arrangement of the invention is shown in FIG. 4. Thissecond arrangement includes the same elements contained in the twosandwiched loops 29 and 34 described above, and accordingly, theseelements are identified with the same name and number. The distinctionfrom the first arrangement is the formation of only a single sandwichedloop 66 which contains the four motor armatures M M M and M and the fourgenerator armatures G G G and G Specifically, first motor 19 connects tothe first generator 20 at a first node 67, first generator 20 connectsthe second motor 23 at a second node 68, second motor 23 connects thesecond generator 26 at a third node 69, second generator 26 connects tothird motor 35 at a fourth node 70, third motor 35 connects to thirdgenerator 36 at a fifth node 71, third generator 36 connects to fourthmotor 41 at a sixth node 72, fourth motor 41 connects to fourthgenerator 42 at a seventh node 73, and fourth generator 42 connects tofirst motor 19 at an eighth node 74 to complete the loop 66. The speedand direction control circuit 51 is connected by a first pair of leads76 across the first generator armature 6,, and its series field coil 21to sense the voltage thereacross, and by a pair of leads 77 across thecommutator field windings 22 and to sense the current therethrough. Asin the first arrangement described above the speed and direction control51 operates in response to the voltage and current feedback informationto control current flow through one of two sets of generator fieldwindings. The first set includes forward field windings 54, 55, 56 and57 associated with the respective generators 20, 26, 36 and 42; and thesecond set includes reverse field windings 58, 59, 60 and 61 alsoassociated with the respective generators 20, 26, 36 and 42. Byenergizing the first field winding set, current flows in one directionin the sandwiched loop 66, while energization of the second fieldwinding set results in reverse current flow and a consequent reversal ofmotor rotation.

The sandwiched loop 66 includes two sets of equalpotential nodes. Thefirst set includes the four odd numbered nodes 67, 69, 71 and 73; andthe second set includes the four even numbered nodes 68, 70, 72 and 74.The four swing drive units 8-11 are damped by connecting dampingresistors between a pair of nodes of the first set and between a pair ofnodes of the second set. Specifically in the embodiment shown, a firstdamping resistor 78 connects the second node 68 to the fourth node 70and a second damping resistor 79 connects the first node 67 to the fifthnode 71. Other damping resistors can be connected between pairs ofequal-potential nodes in one of the two identified sets.

It should be pointed out, however, that there are countless sets ofequal-potential nodes in a sandwiched loop. Specific sets have beenidentified herein because the equal-potential nodes in them are easilyaccessible for attachment of the damping resistor. In addition to thetwo sets already identified, the loop 66 includes, for example, a thirdeasily accessible set of equal-potential nodes at the junction of theseries field winding and commutating field winding of each generator 20,26, 36 and 42. Damping resistors can be attached between any pair ofthese nodes to practice the invention.

In should be apparent to those skilled in the art that the presentinvention applies to drive units using motors and energy convertingmachines other than the d-c motor and d-c generator specificallydescribed herein. For example, the teaching of the present invention canimprove the performance of drive units which use hydraulic motors drivenby hydraulic pumps. The hydraulic motors are connected together seriallywith the hydraulic pumps, in alternate arrangement, to form one or moresandwiched loops. Points in each loop having equal hydraulic pressureunder steady state conditions are thus established and an energydissipating means such as a line with an orifice is connected betweenany two of these points to damp mechanical oscillations. Other means forsuccessfully applying the present, invention to damp mechanicaloscillations in systems driven by a plurality of motors should beapparent from the above description, and reference is therefore made tothe following claims which specifically define the scope of theinvention.

I claim:

1. In the swing drive of an excavating machine having a swing gear and aplurality of swing drive motors, each rigidly connected to said swinggear to impart a swing motion to the excavator when supplied withenergy, and each being subject to oscillations which may occur in saidswing drive by virtue of its rigid connection to said swing gear, theimprovement comprising:

a plurality of generators connected in a sandwiched loop with said swingdrive motors to supply energy thereto and to control said swing drivemotors;

a plurality of equal-potential node sets formed in said sandwiched loopby the interconnection of said swing drive motors and said generators;and damping resistor connected between a pair of equal-potential nodesin one of said sets, said damping resistor having a value chosen toprovide optimal damping which is substantially equal to the equivalentelectrical impedance present across said pair of equal-potential nodes,said damping resistor being responsive to variations in potentialbetween said pair of equal-potential nodes caused by said oscillationsin said swing drive to dissipate energy and thereby damp saidoscillations.

1. In the swing drive of an excavating machine having a swing gear and aplurality of swing drive motors, each rigidly connected to said swinggear to impart a swing motion to the excavator when supplied withenergy, and each being subject to oscillations which may occur in saidswing drive by virtue of its rigid connection to said swing gear, theimprovement comprising: a plurality of generators connected in asandwiched loop with said swing drive motors to supply energy theretoand to control said swing drive motors; a plurality of equal-potentialnode sets formed in said sandwiched loop by the interconnection of saidswing drive motors and said generators; and a damping resistor connectedbetween a pair of equal-potential nodes in one of said sets, saiddamping resistor having a value chosen to provide optimal damping whichis substantially equal to the equivalent electrical impedance presentacross said pair of equal-potential nodes, said damping resistor beingresponsive to variations in potential between said pair ofeQual-potential nodes caused by said oscillations in said swing drive todissipate energy and thereby damp said oscillations.